1. Field of the Invention
The present invention relates to a substrate holding apparatus for holding a substrate as a workpiece to be polished and pressing the substrate against a polishing surface, and more particularly to a substrate holding apparatus for holding a substrate, such as a semiconductor wafer or the like, in a polishing apparatus which planarizes a substrate by polishing the substrate. The present invention also relates to a polishing apparatus having such a substrate holding apparatus, and a polishing method which is carried out by such a polishing apparatus.
2. Description of the Related Art
In recent years, semiconductor devices have become more integrated, and structures of semiconductor elements have become more complicated. Further, the number of levels in multi-level interconnects used for a logical system has been increased. Accordingly, irregularities on a surface of a semiconductor device become increased, so that step heights on the surface of the semiconductor device tend to be larger. This is because, in a process of manufacturing a semiconductor device, a thin film is formed on a semiconductor substrate, then micromachining processes, such as patterning or forming holes, are performed on the semiconductor substrate, and these processes are repeated many times to form subsequent thin films on the semiconductor substrate.
When irregularities of a surface of a semiconductor device are increased, the following problems arise: A thickness of a film formed in a portion having a step is relatively small when a thin film is formed on a semiconductor device. An open circuit is caused by the disconnection of interconnects, or a short circuit is caused by insufficient insulation between interconnect layers. As a result, good products cannot be obtained, and yield tends to be reduced. Further, even if a semiconductor device initially works normally, reliability of the semiconductor device is lowered after long-term use. At a time of exposure during a lithography process, if an irradiation surface has irregularities, then a lens unit in an exposure system is locally unfocused. Therefore, if the irregularities on the surface of the semiconductor device are increased, then this becomes problematic in that it is difficult to form a fine pattern itself on the semiconductor device.
Accordingly, in a process of manufacturing a semiconductor device, it increasingly becomes important to planarize a surface of a semiconductor substrate. The most important one of the planarizing technologies is CMP (Chemical Mechanical Polishing). In a chemical mechanical polishing process, which is performed by a polishing apparatus, while a polishing liquid containing abrasive particles, such as silica (SiO2), is supplied onto a polishing surface, such as a polishing pad, a substrate, such as a semiconductor wafer, is brought into sliding contact with the polishing surface, thereby polishing the substrate.
This type of polishing apparatus comprises a polishing table having a polishing surface constituted by a polishing pad, and a substrate holding apparatus, which is called as a top ring or a carrier head, for holding a semiconductor wafer. When a semiconductor wafer is polished with such a polishing apparatus, the semiconductor wafer is held and pressed against the polishing table under a predetermined pressure by the substrate holding apparatus. At this time, the polishing table and the substrate holding apparatus are moved relatively to each other to bring the semiconductor wafer into sliding contact with the polishing surface, so that a surface of the semiconductor wafer is polished to a flat mirror finish.
In such a polishing apparatus, if a relative pressing force between the semiconductor wafer being polished and the polishing surface of the polishing pad is not uniform over an entire surface of the semiconductor wafer, then the semiconductor wafer may insufficiently be polished or may excessively be polished at some portions depending on a pressing force applied to those portions of the semiconductor wafer. Therefore, it has been attempted to form a surface, for holding a semiconductor wafer, of a substrate holding apparatus as an elastic membrane made of an elastic material, such as rubber, and to supply fluid pressure, such as air pressure, to a backside surface of the elastic membrane to uniformize pressing forces applied to the semiconductor wafer over an entire surface of the semiconductor wafer.
Further, the polishing pad is so elastic that pressing forces applied to a peripheral portion of the semiconductor wafer being polished become non-uniform, and hence only the peripheral portion of the semiconductor wafer may excessively be polished, which is referred to as “edge rounding”. In order to prevent such edge rounding, used a substrate holding apparatus has been used in which a semiconductor wafer is held at its peripheral portion by a guide ring or a retainer ring, and an annular portion of the polishing surface that corresponds to the peripheral portion of the semiconductor wafer is pressed by the guide ring or the retainer ring.
However, the use of the retainer ring is problematic in that the semiconductor wafer held in place by the retainer ring tends to be accidentally dislodged from the substrate holding apparatus during the polishing process, and cannot stably be polished.